Modern vehicle powertrains may include two power sources, such as a combustion engine and a motor. The engine utilizes fuel, while the motor utilizes electrical energy stored in a battery, converting the energy to mechanical energy to propel the vehicle. In a parallel hybrid electric powertrain, in addition to the propulsion power available from the motor, the engine is mechanically connected to the wheels for propelling the vehicle. In a powertrain that is commonly referred to as a series hybrid electric powertrain, the engine is operatively connected to the wheels only via the generator and the motor, and there is no direct mechanical connection between the engine and the wheels.
Extended range electric vehicles may be similar to series hybrid electric powertrains, and typically employ a relatively large battery in order to provide sufficient electrical power to power the motor for propelling the vehicle in an electric-only mode for a relatively long driving range. Some electric vehicles and hybrid electric vehicles are configured such that the battery can be recharged by an off-board power source, and are sometimes referred to as plug-in hybrid vehicles, as they may allow recharging by plugging into a power grid at a home or office. These and other types of hybrid vehicles are capable of operating in an electric-only mode from start to finish of many typical driving trips without the engine ever turning on. Such operation increases fuel economy and decreases vehicle emissions, a large percentage of which occur shortly after engine start, when the engine is running at a relatively low temperature. These benefits of operating in electric-only mode must be balanced against the potential for increased battery wear that may occur as the battery state of charge is reduced to a lower level as the range of operation in the electric-only mode is extended.